A capacitive touch sensor detects a touch point by detecting the capacitance variations of sensor traces caused by the object touch thereon. However, the capacitance variation is usually slight and thus the ambient noise is easy to interfere with the capacitance touch sensor by introducing thereinto via the coupling capacitor between the noise source and the sensor traces. Furthermore, capacitive touch sensors are mostly applied to electronic apparatus such as mobile devices, so it is also important to cope with interference caused by other components of the electronic apparatus. FIG. 1 is a schematic diagram of a capacitive touchpad that includes a capacitive touch sensor 10 as a human-machine interface, on which there are a plurality of sensor traces 20 connected to an analog multiplexer 30 by which to select among the sensor traces 20 to connect to a front-end signal detector 50, and channels 40 between the analog multiplexer 30 and the front-end signal detector 50, whose number determines the number of sensor traces 20 that can be detected at a same time to extract a sensed value ADC. The sensed value ADC determined by the front-end signal detector 50 is for use in further processing or computation, for example, in determining the coordinates, moving distance, moving speed and so on of a touch point. As can be known from this process, the signal ultimately produced by a capacitive touchpad is based on the sensed value ADC provided by the front-end signal detector 50, so accuracy of the sensed value ADC is very important. When the capacitive touch sensor 10 is installed in an electronic apparatus, components of the electronic apparatus often generates a periodic interference noise N which comes into the sensor traces 20 via the coupling capacitor between the noise source and the sensor traces 20 and thus interferes in the signal extracted by the front-end signal detector 50 from the sensor traces 20 and thereby the sensed value ADC generated by the front-end signal detector 50.
For example, referring to FIG. 2, sensor traces 201 and 202 have capacitors C1 and C2 thereon, respectively, and switches 301 and 302 in the analog multiplexer 30 are switched to connect the sensor trace 201 or 202 to an input end mux1 or mux2 of the front-end signal detector 50, to be charged/discharged by a charge pump 501 for modulation to generate a modulated signal mux1 or mux2 that will be demodulated by a switching circuit 502 by switching switches S0˜S3 thereof to generate signals pp and np to be further filtered by a low-pass filter 503 to generate signals ppeak and npeak at output ends. The signals ppeak and npeak will be amplified and converted by other circuits in the front-end signal detector 50 into digital signals which contain the sensed value ADC of the detected sensor trace 201 or 202. FIG. 3 schematically shows the noise coupling, in which the noise N having a fixed period is applied to the sensor trace 201 via a coupling capacitor Ccouple and thus incorporated into the modulated signal mux1 as noise, resulting in an error in detection to the capacitor C1. The noise N may be also transferred via the switching circuit 502 into the low-pass filter 503 and accumulate therein. The most serious effects of the noise N come from its positive edges and negative edges.